There are known in the prior art various dryers all of which make use of the Coanda effect. As is known, a significant of feature of the Coanda effect, which makes nozzles employing the effect especially suitable in dryers is the entrainment in the nozzle stream of a relatively large quantity of the surrounding air.
In most of the nozzle units employing the Coanda effect, the stream of air emerging from the nozzle travels along a curved surface into the space between a pressure plate of the nozzle and the web to be dried. U.S. Pat. No. 3,587,177 shows a dryer of this type which relies on a reduced pressure along the horizontal surface of the pressure plate created by the Bernoulli effect to hold the web close to the nozzle. This increases the possibility of flutter or instability marring the web coating. The structure shown in this patent also has an inherent instability problem at the edge at which the web leaves the airfoil. Attempts have been made to solve this problem.
U.S. Pat. No. 3,711,960 shows a dryer employing a plurality of Coanda nozzle assemblies in which respective upper and lower nozzle assemblies are arranged in staggered relationship above and below the web to be dried so as floatingly to support the web in the course of its movement through the dryer. The patentees suggest that the airfoil surface of each nozzle, which extends parallel to the web should terminte in a sharp edge to release air in a direction of flow along the web.
U.S. Pat. No. 4,414,757 discloses an improved Coanda effect nozzle assembly in which a primary Coanda airfoil nozzle directs air downstream along the face of the pressure plate toward a secondary nozzle of the impingement type for directing air substantially perpendicularly to the airflow produced by the primary nozzle. Among other effects, this secondary nozzle causes the primary air stream to form a pad of air or an increase in static pressure in the primary flow zone which assists in keeping the web spaced from the pressure plate.
U.S. Pat. No. 3,549,070 discloses a web flotation nozzle which uses a dual discharge slot. This arrangement not only requires a relatively large amount of air but also is subject to instabilities which may result from any mismatch in the two opposing jets of air.